This invention relates to a measuring circuit for measuring the resistance value of a resistor, which circuit comprises:
a capacitor, PA1 a first resistor and a second resistor which can be coupled to the capacitor for charging the capacitor, thereby forming a capacitor voltage PA1 discharge means for discharging the capacitor, PA1 a first reference voltage source for generating a first reference voltage, PA1 first comparison means for comparing the capacitor voltage with the first reference voltage and for generating a first detection signal when the capacitor voltage passes the first reference voltage, PA1 time measurement means for measuring a first time interval which, after discharging of the capacitor by the discharge means, terminates upon the occurrence of the first detection signal during charging of the capacitor via the first resistor, and for measuring a second time interval which, after discharging of the capacitor by the discharge means, terminates upon the occurrence of the first detection signal during charging of the capacitor via at least the second resistor. PA1 a second reference voltage source for supplying a second reference voltage which lies between the first reference voltage and the capacitor voltage immediately after discharge of the capacitor, PA1 second comparison means for comparing the capacitor voltage with the second reference voltage and for generating a second detection signal when the capacitor voltage passes the second reference voltage, and in that PA1 the first time interval and the second time interval start upon the occurrence of the second detection signal. PA1 a voltage divider with a series arrangement of a third resistor and a fourth resistor, PA1 a fifth resistor, and PA1 switching means for connecting the fifth resistor and the third resistor in parallel before the occurrence of the second detection signal and for connecting the fifth resistor and the fourth resistor in parallel after the occurrence of the second detection signal.
The invention also relates to a thermal appliance, an electrical thermometer and a cold-generating appliance including such a measuring circuit.
Such a measuring circuit is known from U.S. Pat. No. 4,910,689. After it has been discharged the capacitor is charged via the first resistor and the time is measured which is required to charge the capacitor until the capacitor voltage has become equal to the first reference voltage. This measurement is the first measurement and the measured time is the first time interval. Subsequently, the second resistor is arranged in parallel with the first resistor and the capacitor is discharged again. After this, the capacitor is charged again but now via the parallel-connected first resistor and second resistor, and again the time is measured which is needed to reach the first reference voltage. This measurement is the second measurement and the measured time is the second time interval. The ratio between the first and the second time interval is equal to the ratio between the resistance value of the first resistor and the resistance value of the parallel-connected first resistor and second resistor. The resistance value of either the first or the second resistor is known, which is the reference resistance, so that the value of the other resistor, i.e. the unknown resistance to be measured, can be calculated from the ratio.
In the known measuring circuit the second resistor is arranged in parallel with the first resistor during the second measurement. The first resistor is connected permanently to the capacitor. However, it is also possible to charge the capacitor exclusively via the second resistor during the second measurement. The first resistor is then disconnected during the second measurement. The value of the unknown resistance can then again be derived from the ratio between the measured time intervals.
The unknown resistance may be a temperature-dependent resistance, for example, a thermistor or an NTC (negative temperature coefficient) resistor. The measured resistance is then a measure of the temperature of the resistor. In that case the measuring circuit is very suitable for use in electrically heated appliances such as a flat-iron, coffee maker, electric kettle, deep fryer, roaster, cook-top, oven, grill, hot-plate, room-heating appliance, radiant heater, fan heater, hair dryer, hair curler, bread toaster, sandwich toaster, electric blanket and the like, electrical thermometers and cold-generating appliances such as an icemaker, food processor, refrigerator, deepfreezer, air conditioner and the like.
During the discharge of the capacitor two currents will flow through the discharge means. Firstly, a discharge current which decreases to zero as a result of the short-circuit of the capacitor. Secondly, a substantially constant charging current whose magnitude at the beginning of the first measurement is mainly determined by the first resistance and at the beginning of the second measurement by the parallel-connected first and second resistance. Owing to the finite impedance of the discharge means the charging current produces a voltage drop across the discharge means, which voltage drop will be left across the capacitor as a residual voltage at the beginning of the first or the second measurement when the discharge path of the discharge means is opened. The residual voltage influences the time required to charge the capacitor to the first reference voltage and hence the lengths of the first and the second time intervals. During the second measurement the first and the second resistor are in parallel, so that the charging current at the beginning of the second measurement is larger than at the beginning of the first measurement. Consequently, the residual voltages across the capacitor also differ per measurement. Moreover, if the unknown resistor is an NTC resistor this difference in residual voltages also varies as a function of temperature. These residual voltages consequently result in an inaccurate measurement.